A peak of a communication signal represents the greatest instantaneous amplitude, magnitude, or power level exhibited by the communication signal within some period of time. The average of a communication signal represents the average amplitude, magnitude, or power level of the communication signal over that same period. The peak is greater than the average, and the ratio of the peak power to the average power (PAPR) is a parameter of interest to communication system designers.
As PAPR increases, meeting a transmitting unit's design goals becomes increasingly difficult. A transmitting unit's power amplifier is desirably as linear as possible when used to amplify communication signals modulated in accordance with many modern modulation formats. But linearity is achieved only so long as the amplitude of a communication signal remains beneath some maximum level. If the communication signal's instantaneous power exceeds this maximum level, non-linear amplification results, causing the spectrum of the communication signal to grow and exceed regulatory limitations imposed on the transmitting unit. Accordingly, the communication signal's instantaneous power should be kept below this maximum level.
Transmitting units typically operate under economic constraints which prevent the maximum instantaneous power level for linear operation to be at a high level. Significant costs are typically involved in providing power amplifiers and power amplifier biasing systems which support linear operation up to the high level.
If the average power level of the communication signal is far beneath a modest and economically practical maximum level, one or more of several undesirable consequences result. The power amplifier may operate inefficiently and consume more power than desired for the communication performance achieved. The distance over which the communication signal may be successfully received may become diminished. Or, a less efficient modulation may be used in order to effect communications, resulting in slower data rates for longer durations in order to transmit a given amount of data. Transmitting a given amount of data over a longer duration increases power consumption. Since many transmitting units are battery operated, the consumption of excessive power is a particularly undesirable design feature because excessive power consumption leads to the use of undesirably large batteries and/or frequent battery recharging.
The communication signals amplified by the transmitting units' power amplifiers and compatible with many modern communication standards tend to exhibit high PAPR levels. This problem is particularly acute in connection with communication signals modulated in accordance with modern communication standards configured to implement OFDM and OFDMA formats. Accordingly, modern transmitting units tend to take steps to reduce the PAPR of the communication signal prior to amplification in a power amplifier.
A variety of techniques for PAPR reduction have been developed. When a communication signal's peak is maintained near the maximum level for linear operation of the power amplifier and when PAPR has been reduced, the average power level may then be increased, causing improved communication performance. But to be effective at PAPR reduction, the techniques used to reduce PAPR should avoid spectral regrowth to remain within regulatory limitations and introduce as little noise into the communication signal as possible. The introduction of noise through PAPR reduction reduces communication performance improvements achieved through operating at a higher average power level. If too much noise is introduced, any communication performance improvement will be entirely counteracted. One particularly effective technique is taught in “Method and Apparatus for Adaptively Controlling Signals,” U.S. Publication No. 2007/0254592, filed 27 Apr. 2006, invented by the inventors of the present invention, and incorporated by reference, in its entirety, herein.
But it is also desirable that PAPR reduction be performed in a way that consumes as little power as possible. The efficient use of power is almost always a desirable goal, and since many modern transmission units are battery operated this desirable goal takes on added importance. Moreover, in some applications the efficient use of power may be of such importance that even highly effective PAPR reduction techniques are not practical unless they are also particularly efficient at consuming power. Accordingly, a need exists for a transmitting unit that reduces PAPR in a particularly effective manner and in a manner that consumes as little power as possible.